specifications say that both cpus run up to 1600mhz ram but that doesnt mean that if you put 1866 or 2100 they wont run better.
this is like to buy ferrari and to drive it only on 1st gear.
kids dont care about perofmance now but after an year when they cant play some game they will start whine that they whant new pc.
you cant have sli from 650 and 660. my idea was if it is possible to return 650 and 660 to the shop and take 2x770 (1 for each rig) paying only difference. but since you plan to upgrade in futer then cards you have now will hold it till upgrade.
NAS mean Network Attached Storage. this is separe computer specialy designed to run only hdds with or w/o raid. this is like file server but in smaller scale. on those devices you can setup user pofiles and access permisions (you can setup what your kids can see and what cant), store backups of ssds, stream movies to TV etc.
here
http://www.techpowerup.com/reviews/?category=NAS&manufacturer=&pp=25&order=date you can check reviews of nas drives.
some of new models arnt reviewed yet. you can search for synology DS213j, qnap TS-212, and i think TS 213 is avialble as well, asustor also has 2 bay NAS but cant remeber model.
point here is to remove old hdds from your rigs and put them in separete pc (NAS). this way they wont slow down your rigs.
what size disks you put in your rigs depends on how you are going to use them.
also if you havnt installed ssds so far you can find good starter guid here
http://www.ocztechnologyforum.com/forum/showthread.php?110371-ABC-GUIDE-New-Build-2013 it starts with ocz drives but all other things like os install and tweeks (except firmware update) are valid for all ssds